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What is stainless steel?

Today, iron is the most widely-used metal. It’s durable, versatile, and abundant in the Earth’s crust (making it cheap). However, in its pure form, iron is very susceptible to oxygen — it rusts rapidly. Stainless steel provides a solution to this problem.

Image via Pixabay.

The world as we know it wouldn’t be possible without stainless steel. It’s a material that combines strength, flexibility, and durability at an affordable cost. This alloy makes an appearance in everything, from high-rises and high-performance cars to spoons and baby monitors. To quite a large extent, our world is built on stainless steel. So let’s learn more about it.

What is stainless steel?

‘Stainless steel’ is a generic, umbrella term, that denotes a wide range of metal alloys — cocktails of metals — based on iron. Like all other types of steel, it also contains carbon.

It has excellent resistance to corrosion (oxidation or rusting), is relatively non-reactive with most chemicals, has high durability, and good hygienic properties. It sees wide use today in products ranging from cutlery to medical devices to construction materials.

Aesthetically, stainless steel is a lustrous, silvery metal, that can take a very high polish. From a practical point of view, stainless steel is a strong and highly resilient material; its exact properties depend on the composition of the alloy, but it can be tailored to suit a wide range of needs, having the potential to be highly flexible, resistant to scratching, mechanically tough, or any other property needed in a certain application. It can be recycled practically forever, as its recovery rate during recycling is close to 100%

Although it is more difficult and expensive to produce than iron metal, stainless steel has practically replaced iron in all except the most specialized cases due to the advantages it holds over the un-alloyed metal. Almost all ‘iron’ products you’ve encountered in your life were made from stainless steel.

What is stainless steel made of?

Molten metal pouring from ladle. Image credits Goodwin Steel Castings / Flickr.

Stainless steel differs from other types of steel through the addition of a handful of elements to the mix. The exact elements added vary with alloy type but, as a rule of thumb, stainless steels contain chromium (Cr), in quantities ranging from 10.5 to 30% by weight.

Chromium is what gives these alloys their high resistance to corrosion. As it interacts with corrosive agents in the air, chromium forms a passive layer — a ‘film’ of chromium oxide — on the metal’s surface which protects the alloy. Oxygen and moisture cannot penetrate this film, so it protects the iron throughout the body of the steel from rusting.

Other elements that are added to stainless steel include non-metals such as sulphur, silicon, or nitrogen, metals such as nickel, aluminium, copper, or more exotic metals such as selenium, niobium, and molybdenum. Although the exact composition of the alloy is decided based on its desired properties — each element added in, and their proportion, changes the characteristics of the alloy — some of the most commonly-seen extra elements in stainless steel alloys are nickel and nitrogen. These improve its hardiness and ability to resist corrosion in certain conditions, but also increase its price per pound.

There are currently over 100 types (known as ‘grades’) of stainless steel being produced and used, each with its own ISO number, many of them for specialized applications. The most common five types are known as ‘austenitic’, ‘ferritic’, ‘martensitic’, ‘duplex’, and ‘precipitation hardening’ steels.

  • Austenitic stainless steels are the most widely used grade. They have very good resistance to corrosion and heat, offering good mechanical properties over a wide range of temperatures. It’s used in household goods, industrial applications, in construction, and in decorations.
  • Ferritic stainless steels have lower mechanical resitance — they resemble mild steels in strength — but are better able to resist corrosion, heat, and are harder to crack. Any washing machine or boiler you have at home are probably made of ferritic steel.
  • Martensitic stainless steels are much harder and stronger than their peers, but they’re not as able to withstand corrosion. This is the type of steel that makes high-grade knives, and is also used for turbine blades.
  • Duplex stainless steel is a mixture of austenitic and ferritic steels, and their properties are, similarly, a middle-ground between these two grades. As a rule of thumb, it is used in applications where both strength and flexibility are required, and corrosion resistance is a plus; shipbuilding is a prime example.
  • Precipitation hardening stainless steels are a subclass of alloys, somewhere in the overlap between martensitic and austenitic steels. They offer the best mechanical properties of the lot (they have very high material strength), due to the addition of elements such as aluminium, copper, and niobium.

What is stainless steel used for?

Stainless steel facade. Image credits Dean Moriarty.

With a material as versatile as stainless steel, it’s hard to cover all its uses in any detail. Suffice to say, it’s used in virtually all goods and applications where strength, flexibility, good looks, and hygiene are required, for relatively low cost, and weight is not a huge concern.

Household goods and appliances make heavy use of stainless steel, especially kitchenware or other products meant to come into contact with water. Knives and cutlery, home appliances such as washing machines, bathroom fixtures, piping, cookware make use of stainless steel due to its resistance to corrosion, its good looks, ease of washing, and high durability. Various grades of stainless steel are used depending on the intended role and usage of each product.

Medical tools also make ample use of stainless steel. Things like surgical and dental instruments, scissors, trays, and a wide range of other medical-use objects are made from this alloy. Here, it is the chemical inertness and corrosion resistance of stainless steel that is most important. Medical devices also contain stainless steel, in particular structural elements and coverings, due to their strength and ease of cleaning. Medical implants, such as those used in knee or hip replacement surgery, are also made of stainless steel.

Stainless steel is also used in the construction of vehicles, mostly ships, trains, and cars. Aircraft manufacturers tend to prefer aluminium alloys, as they are more lightweight. That being said, stainless steel is essential in the production of aircraft frames and various structural elements of the landing gear. For all vehicles, however, stainless steel combines good mechanical properties with high longevity (due to its resistance to corrosion), making for durable and long-lived parts.

Construction and architecture are two further domains that love stainless steel. The combination of strength and high chemical inertness makes this alloy ideal for structural elements in buildings such as skyscrapers, or in exposed elements, such as fire escapes or service ladders.

Jewelry manufacturers also employ stainless steel in their products, where it’s preferred due to its hypoallergnic properties (it doesn’t trigger metal allergies).


Stainless steel, today, is an indispensible alloy. Our societies depend heavily on it, using it in everything from tiny knicknacks around the house to the mightiest skyscraper. Its unique combination of strength, longevity, and relatively low cost makes it so that, most likely, stainless steel won’t be replaced anytime soon.

Swedish company produces the first slab of steel that didn’t require any coal

Engineers from the SSAB steel-making company have unveiled the world’s first piece of steel cast without burning any coal or fossil fuel. Instead, they used hydrogen to power the process.

The first steel produced using HYBRIT technology. Image credits SSAB

Metalworking and coal burning have been entwined for as long as humanity has been using metals. Coal is a very good source of energy, providing the heat necessary to refine and process most metals. But it is also a source of carbon, a critical chemical in the production of steel, and the compound that allows us to turn metal ores (usually oxides) into actual metals (by leaching out the oxygen).

For most of our history, this wasn’t that much of an issue. Coal smoke is definitely not healthy for you or anyone living near the smeltery or ye olde blacksmith, but overall production of metals was limited in scope — so the environment could absorb and process its emissions.

Today, however, the sheer scale at which we produce metals means that this process has a real impact on the health of the world around us. However, new technology could uncouple the process from coal, and pave the way towards ‘green’ metals. Engineers from the international, Sweden-based steel-making company SSAB have showcased the process, which relies on hydrogen instead of coal to produce the necessary temperatures.

Transition metal

“The first fossil-free steel in the world is not only a breakthrough for SSAB, it represents proof that it’s possible to make the transition and significantly reduce the global carbon footprint of the steel industry,” said Martin Lindqvist, SSAB’s president and CEO, for CNBC.

The “hybrid process” used by SSAB uses hydrogen as fuel to produce the required energy, instead of the traditional approach of burning coal. This process, called HYBRIT (Hydrogen Breakthrough Ironmaking Technology), uses electricity produced through renewable means to produce hydrogen, which is in turn burned to generate heat. Although there is burning involved, it doesn’t produce any pollution — in fact, the only end product is water.

HYBRIT can be used both for the production of iron pellets — the main raw material used by steel foundries — and in the carbon purification process, which is the step that transforms iron into steel. The first piece of HYBRIT steel was produced for the Volvo Group and is going to become a part of the company’s fleet of trucks. A candleholder was also machined from this steel as proof that its mechanical properties are the same as regular steel produced by SSAB.

The candle holder. Image credits SSAB.

“The candle holder, with its softly pleated rays beaming out from the candle, symbolizes the light at the end of the tunnel. It is a symbol of hope. It truly is a piece of the future,” says Lena Bergström, who designed the item.

The steel industry today accounts for roughly 9% of global carbon dioxide emissions, and demand for (as well as production of) steel is steadily increasing.

SSAB developed the process in the context of a joint venture with the government-owned utility Vattenfall and Swedish mining company LKAB. The steel was processed in a pilot plant in the north of Sweden, and full-scale production capability is not expected for another five years or so, according to Reuters. The slab of metal produced so far marks the culmination of over 5 years of research and development of the HYBRIT process.

“The goal is to deliver fossil-free steel to the market and demonstrate the technology on an industrial scale as early as 2026,” a statement form SSAB explained.

Persians were making proto-stainless steel 1,000 years ago

Stainless steel is generally considered to be a recent innovation, but according to archaeological evidence, ancient Persians came very close to producing this alloy ten centuries ago.

A piece of an ancient crucible containing an embedded chunk of slag. Image credits: Rahil Alipour/UCL Archaeology.

Stainless steel is essentially a steel-chromium alloy. Compared to ‘normal’ steel, stainless steel has anti-corrosion and heat-resistant properties, as researchers in the early 19th century found when they first developed this material.

“We analysed archaeological finds from the 11th c. CE site of Chahak in Iran, showing the intentional and regular addition of chromium mineral to the crucible charge, resulting in steel containing around 1% chromium,” the authors note in the new study.

While this 1% is lower than modern stainless steel, it’s still enough to indicate that it wasn’t a coincidence or an error. So it’s not exactly stainless steel, but rather a precursor to it — which is remarkable, as the ancient Persians had this technology centuries before modern society.

The steel was presumably used to produce weaponry, such as swords, daggers, and armor, but according to the archaeologists’ analysis, it also contained phosphorous, which made them a bit more brittle.

The site where the discovery was made is also intriguing. Now a small village in Iran, Chahak used to be an important hub for steel production, and it’s the only site in the area with evidence of crucible steel-making.

A large chunk of steel trapped in crucible slag. Image credits: Rahil Alipour/UCL Archaeology.

Crucible steel is made by melting cast iron (and sometimes other steel) with organic matter, sand, or other materials. It’s impossible to melt iron on an open coal fire, but cast iron has a carbon content of 2%, which lowers its melting point. By soaking the iron for a long time, the carbon content can be reduced, thus producing a purer type of steel. Other materials can also be added to enhance the properties of the steel and, overall, crucible steel was superior to other types available at the time. It’s also this technique that, with subsequent forge and polish, produced the wootz steel used in Damascus swords.

The Persians weren’t the only ones to make crucible steel (several other cultures, including the Vikings, also did it), but this is the first example of chromium steel.

Researchers were directed to search for chromium steel by an ancient manuscript by the Persian polymath Abu-Rayhan Biruni, who was a pioneer in fields as diverse as geodesy and comparative religion — many consider him to be the world’s first anthropologist. Biruni’s document, which dates back to the 10th or 11th century CE is called “al-Jamahir fi Marifah al-Jawahir” (translated to “A Compendium to Know the Gems”). It offers an indication on how to forge crucible steel, but it also includes a mysterious compound that Biruni calls rusakhtaj (meaning “the burnt”). Researchers have wondered for a while what this rusakhtaj could be — it has now been identified as chromite sand.

The Persians were also sneaky. Archaeologists found manuscripts where Chahak is referenced as a place where beautiful blades were sold for a high price, but were then found to be quite fragile. This happened because the Persians also added phosphorous to the crucible steel — this lowered the melting point and made it easier to produce steel, but it also made the resulting product more brittle.

The practice of adding chromium is so unique that researchers now believe it could be used to differentiate steel that came from Chahak. Museums around the world have Persian steel artifacts and the chromium content could be used to trace back their origin.

The study has been published in the Journal of Archaeological Science.

By adding fibers, scientists have turned a soft gel into a material tougher than many metals. Credit: Hokkaido Uni.

Adding fibers to hydrogel, a soft material mostly made of water, makes it 5 times tougher than steel

By adding fibers, scientists have turned a soft gel into a material tougher than many metals. Credit: Hokkaido Uni.

By adding fibers, scientists have turned a soft gel into a material tougher than many metals. Credit: Hokkaido Uni.

Hydrogels are made of a network of polymer chains that are hydrophilic. For this reason, this material can absorb up to 90% of its weight in water making the hydrogel highly flexible, mimicking natural tissue. On the flipside, hydrogels aren’t very strong. However, Japanese researchers from Hokkaido University have found a way to make very strong hydrogels by introducing fibers into their composition without compromising too much flexibility. The fiber-reinforced hydrogels are five times tougher than steels according to tests ran in the lab.

Due to its properties, hydrogel is used for medical purposes in tissue engineering as well as sustained-release of drug delivery system or rectal diagnoses. It also has the potential to become a very useful structural material if only it could be coaxed to be stronger for long-term use.

The Japanese team led by Dr. Jian Ping Gong looked at the problem and decided to go with a time-honored approach: just mix and match with some other material to create a composite material that has all of the desired properties. For instance, mud is mushy but add some straw to it and leave it to bask in the sun for a while and you have some pretty functional solid bricks. It’s how people used to make their homes for thousands of years.

Likewise, the researchers added glass fiber fabric to polyampholyte (PA) hydrogels that contained high levels of water. Though the fibers were no bigger than 10μm in diameter, slightly thinner than the human hair, the resulting composite material proved to be bendable and very strong at the same time.

Scanning Electron Microscopy (SEM) images of the fiber-reinforced hydrogels. The polymer matrix (arrows) filled the interstitial space in the fiber bundles and connected the neighboring fibers. (Huang Y. et al., Advanced Functional Materials, January 16, 2017)

Scanning Electron Microscopy (SEM) images of the fiber-reinforced hydrogels. The polymer matrix (arrows) filled the interstitial space in the fiber bundles and connected the neighboring fibers. (Huang Y. et al., Advanced Functional Materials, January 16, 2017)

Lab tests show the fiver-reinforced hydrogels are 25 times tougher than glass fiber fabric and 100 times tougher than unaltered hydrogels. That makes it 5 times tougher than carbon steel. The researchers measured the energy required to destroy them when assessing the materials’ strength.

It’s not certain yet but the Japanese researchers think this high toughness is due to an increase in dynamic ionic bonds between the fiber and hydrogels, as well as within the gels themselves.

The manufacturing process is quite simple — you only have to immerse the fabric in PA precursor solutions for polymerization — and should prove scalable.

“The fiber-reinforced hydrogels, with a 40 percent water level, are environmentally friendly,” Gong said in a statement. “The material has multiple potential applications because of its reliability, durability and flexibility. For example, in addition to fashion and manufacturing uses, it could be used as artificial ligaments and tendons, which are subject to strong load-bearing tensions.”

The same process, reported in the journal Advanced Functional Materials should be able to make other polymers tougher, like rubber.

slips steel

Coating makes steel stronger and squeaky clean

Used in everything from skyscraper girders, automobiles, and appliances to thumb tacks and paper clips, steel is one of the world’s most vital materials. While there’s been a great amount of research invested into steel, most of it has concentrated on making various grades of steel, with little focus on the surface itself. Understanding that there’s a great interest and need for steel surfaces that can stay clean and don’t corrode under harsh environmental conditions, a group of material scientists at Harvard have come up with a squeaky clean coating that does just that.

slips steel

Source: Aizenberg Lab/Harvard SEAS

Super steel

Called the Slippery Liquid-Infused Porous Surfaces (SLIPS), the surface coating is heralded as the most  durable anti-fouling and anti-corrosive material to date. More specifically, it’s a  nanoporous  grown directly on the steel through an electrochemical technique – a standard manufacturing procedure that doesn’t require millions worth of new tech to be deployed.

SLIPS isn’t uniformly applied, but rather in  ultrathin film of hundreds of thousands of small islands. This proved to be important, since the steel doesn’t suffer mechanical degradation if one of the islands breaks. This means that the resulting steel carries both repellent and abrasive properties at the same time, which was impossible until now.

Accelerated corrosion test, in which unmodified stainless steel (300 grade) (right sample)and the lower part of the TO-SLIPS sample with a 600-nm-thick porous TO film on steel (left sample)were exposed to very corrosive Glyceregia stainless steel etchant. (a-h) Images show corrosion evolution as a function of contact time.

Accelerated corrosion test, in which unmodified stainless steel (300 grade) (right sample)and the lower part of the TO-SLIPS sample with a 600-nm-thick porous TO film on steel (left sample)were exposed to very corrosive Glyceregia stainless steel etchant. (a-h) Images show corrosion evolution as a function of contact time.

To test the material, engineers scratched the steel coated surface with everything from tweezers, to screwdrivers, and even pummeled hundreds of heavy beads. When it was tested against water, corrosive materials and even bacterial infested sludge, all of the liquids were repelled from the steel. Moreover, the resulting steel proved to be stronger sans the coating.

“Our slippery steel is orders of magnitude more durable than any anti-fouling material that has been developed before,” said Aizenberg. “So far, these two concepts – mechanical durability and anti-fouling – were at odds with each other. We need surfaces to be textured and porous to impart fouling resistance but rough nanostructured coatings are intrinsically weaker than their bulk analogs. This research shows that careful surface engineering allows the design of a material capable of performing multiple, even conflicting, functions, without performance degradation,” said Joanna Aizenberg, the Amy Smith Berylson Professor of Materials Science and core faculty member of the Wyss Institute for Biologically Inspired Engineering at Harvard University.

The SLIPS technology for preventing biofilm formation as compared to a Teflon coated surface. (Photo courtesy of Joanna Aizenberg and Tak-Sing Wong.)

The SLIPS technology for preventing biofilm formation as compared to a Teflon coated surface. (Photo courtesy of Joanna Aizenberg and Tak-Sing Wong.)

According to Philseok Kim, co-author of the paper, the SLIPS coating will prove appealing in the biomedical industry where durable, but extra clean surgical equipment is required. Of course, applications where bacterial sludge is rampant on steel surfaces will definitely benefit. Take ship hulls for instance where microorganisms like barnacles and algae force companies and navies to constantly cleanup and apply anti-fouling paints. Also bio 3D printers that use sticky, viscous organic materials instead of polymers could use the anti-fouling, but durable steel coating for its nozzles.

Then there’s the ubiquitous problem of freezing surfaces. The aviation industry spends millions of dollars and countless hours spraying deicing fluid on the wings of planes as they sit waiting on wintery runways. SLIPS could easy solve this problem by repelling ice and water simply using gravity. That’s because it can be applied to other metals too, not just steel. The group tested aluminum refrigeration fins coated with SLIPS in 2013 at -10 degrees Celsius and 60 percent humidity, and the technology significantly outperformed typical “frost-free” cooling systems in terms of preventing frost from forming over time.

“This research is an example of hard core, classic material science,” said Aizenberg. “We took a material that changed the world and asked, how can we make it better?”

 

Modern Blacksmithery: forging a 320 Layer Damascus Steel Blade

In the Middle Ages, Blacksmiths were highly regarded, and this was one of the most active industries. Nowadays, with modern technology, blacksmiths are all but extinct; yet some of them are still forging, working on spectacular blades. Here is such an example:

Damascus steel was a type of steel used in Middle Eastern swordmaking. These swords are characterized by distinctive patterns of banding and mottling reminiscent of flowing water. Blades with Damascus steel are especially resilient, and the reputation of this steel has given rise to many legends, such as the ability to cut through a rifle barrel or to cut a hair falling across the blade.

A study conducted in Germany in 2006 revealed that Damascus steel contains nanowires and carbon nanotubes, generated through the forging process. The team of researchers was based at the Technical University of Dresden and they used x-rays and electron microscopy to examine Damascus steel discovering the presence of cementite nanowires.

A Damascus Blade

A Damascus Blade